This invention relates to compositions of matter for use in making electric cables and to cables in which they are used. More particularly, it relates to compositions for use as xe2x80x9csemiconductingxe2x80x9d dielectric shield (also called core shield, dielectric screen or core screen) materials in power cables with crosslinked polymeric insulation, primarily at xe2x80x9cmediumxe2x80x9d voltages, say from around 10 kV to 75 or perhaps 100 kV.
While some customers desire to have polymeric cables in which the dielectric screen is securely bonded to the insulation so that stripping is only possible by use of a cutting tool that removes a little insulation with the shield, because they believe that this minimises risk of electrical breakdown at the interface, others prefer a xe2x80x9cstrippablexe2x80x9d shield with limited adhesion to the insulation so that it can be peeled cleanly away (generally after cutting xe2x80x9ctramlinesxe2x80x9d part-way through its thickness) without removing any insulation. Current strippable screen compositions for use over insulation of crosslinked polyethylene (XLPE) or one of the ethylene copolymer rubbers (EPR or EPDM, the latter incorporating a diene comonomer to provide unsaturation) are usually based on an ethylene/vinyl acetate copolymer (EVA) rendered conductive with an appropriate type and amount of carbon black: the required peel characteristics can be obtained by selecting an EVA with a high enough vinyl acetate content, with or without using a nitrile rubber as an adhesion-adjusting additive. Formulations using these additives (e.g. Ongchin U.S. Pat Nos. 4,246,023 and 4,246,142, Burns et al European patent 0,420,271B; Kakizaki U.S. Pat. No. 4,412,938 and Jansson U.S. Pat. No. 4,226,823) are expensive proprietary material and in the present applicants"" experience do not sufficiently avoid a requirement to use EVA""s of relatively high vinyl acetate content to achieve the optimum adhesion level, with the result that all the strippable screen compositions in general commercial use are more rubbery than is desirable.
Many alternative adhesion-adjusting additives have been proposed, for example waxy aliphatic hydrocarbons (Watanabe et al U.S. Pat. No. 4,993,107); low-molecular weight ethylene homopolymers (Burns Jr U.S. Pat. No. 4,150,193); various silicone compounds (Taniguchi U.S. Pat. No. 4,493,787); chlorosulfonated polyethylene, ethylene-propylene rubber, polychloroprene, styrene-butadiene rubber, natural rubber, polyester rubber, and polyurethane rubber (all in Jansson U.S. Pat. No. 4,226,823); but none of these, except paraffin waxes, seems to have found commercial acceptance.
This invention is based on the discovery of a new and unexpected class of adhesion-adjusting additive which surprisingly allows shield compositions to be formulated, if desired, utilizing EVA""s of lower vinyl acetate content for a given level of adhesion, and thus to make strippable shields that are less rubbery and thus easier to process than current formulations utilising nitrile rubbers.
The composition of matter in accordance with the invention comprises:
a base polymer which is a copolymer of ethylene with a mono-unsaturated ester;
conductive carbon black in an amount to give the composition an electrical resistivity below 500 om, and
as an adhesion-adjusting additive, 1-20% by weight of the base polymer of a copolymer which consists essentially of ethylene and a comonomer having in its molecule at least one carbon-oxygen dipole, said copolymer having a molecular weight less than 5 20,000 Daltons but a Mettler drop point higher than 30xc2x0 C.
The invention includes an electric power cable having at least one conductor, insulation selected from the group consisting of cross-linked polyethylene and the ethylene copolymer rubbers, a dielectric shield and a surrounding jacket, said dielectric shield being of the composition specified in the preceding paragraph.
The invention also includes the use of a copolymer which consists essentially of ethylene and a comonomer having in its molecule at least one carbon-oxygen dipole, said copolymer having a molecular weight less than 20,000 Daltons but a melting point higher than 30xc2x0 C., as an adhesion-adjusting additive in strippable shield composition for electric cables comprising a base polymer which is a copolymer of ethylene with a mono-unsaturated ester and a conductive carbon black in an amount to give the composition an electrical resistivity below 500 Am.
Preferably the base polymer is an EVA, but the invention is also applicable for base polymers such as ethylene-ethyl acrylate, ethylene-methyl acrylate and ethylene-methyl methacrylate copolymers and ternary (or higher-order) copolymers containing relatively small amounts of at least one additional monomer. Whereas for prior-art compositions in which the additive is a nitrile rubber, an EVA with a vinyl acetate content of at least 33% and an additive content of about 30% would be recommended for use with crosslinked polyethylene insulation (or an EVA with a vinyl acetate content of 40 if the additive level were only 50i), in the practice of the present invention a vinyl acetate content of about 33% is often satisfactory in terms of adhesion with an additive content of about 5% and this is preferred for giving the composition less rubbery physical properties; similarly for use with insulation based on EPR or EPDM, in the practice of the present invention an EVA content of about 40% is often sufficient and preferred at an additive level about 5%.
The invention does not require alteration to current practice regarding the types and quantity of carbon black to be used, and conventional types and proportions may be used. Preferably the resistivity of the composition is in the range from 0.1 to the maximum of 500 om and preferably between 5 and 100 om.
The carbon-oxygen dipole in the adhesion-adjusting additive may be a Cxe2x80x94O single-bond (ether) dipole, a Cxe2x95x90O double-bond (carbonyl) dipole or a C:Oxe2x80x94O ester dipole, and more than one such dipole of the same or different kinds may be present in the comonomer molecule. Thus the comonomer molecule may, inter alia be vinyl acetate, ethyl acrylate, methyl- or ethyl-methacrylate, maleic anhydride and carbon monoxide. Monomers with free carboxylic acid groups are not recommended because their use might entail a risk of introducing ionic contaminants into the composition and from there into the insulation. The more polar comonomers and most especially vinyl acetate are preferred, partly because, as a general rule, a smaller proportion of the comonomer will be needed.
The proportion of the adhesion-adjusting additive required naturally varies with its polarity and other characteristics, but will typically be in the range from about 1 to about 20xe2x80x94preferably in the lower part of this range, say about 2-10%, for the more polar additives such as the EVA""s. It should be noted that these additives are generally less polar than the base polymer, in contrast to NBR which is much more polar, so that this observation is contrary to any supposition that the additives function by incompatibility with the base polymer.
The invention includes an electric power cable having at least one conductor, insulation selected from the group consisting of cross-linked polyethylene and the ethylene copolymer rubbers, a dielectric shield and a surrounding protective layer (such as a polymeric jacket and/or a layer of metal wires), said dielectric shield being of the composition previously defined.
Ethylene copolymer rubbers for use in the cable insulation include conventional EPR and EPDM rubbers, but also include copolymers with higher olefins (such as octene) that have recently become available through the application of xe2x80x9csingle-sitexe2x80x9d metallocene catalysts.